1. Field of the Invention
The present invention relates to a hybrid working machine that uses engine power and electric power.
2. Description of the Related Art
In a hybrid working machine (for example, an excavator), a hydraulic pump that drives a hydraulic actuator and a power machine that performs a generator function and a motor function (usually a generator/motor) are connected to an engine. An electric storage device is charged using the generator function of the power machine, and the discharge power of the electric storage device makes the power machine perform the motor function. The motor function assists driving of the hydraulic pump (see Japanese Unexamined Patent Application Publication No. 2001-12274).
In a fully hydraulic excavator (hereinafter referred to as an ordinary excavator), the maximum input of the hydraulic pump is set equal to or lower than the maximum output of the engine so that the engine may not be overloaded. That is,maximum pump input<maximum engine output.
In contrast, in a hybrid machine that uses the engine power and the assistance power as described above, the maximum output of the engine is set to be lower than that of an ordinary excavator with consideration of the assistance power. (Usually, the maximum output of the engine is set at the average power. Hereinafter, this case is described.) That is,maximum pump input>maximum engine output.
The pump input is defined as (load pressure)×(pump flow rate). In a hybrid working machine, the pump input is shared by the engine output and the assistance output of the power machine.
Regarding control of the pump flow rate in an ordinary excavator, two methods are used in combination. One is a method of controlling the pump flow rate in accordance with the operation amount of operation means for operating the hydraulic actuator (hereinafter referred to as “lever operation amount”) as illustrated in FIG. 6 (flow rate control). The other is a method of controlling the flow rate in accordance with a load pressure (pump pressure) as illustrated in FIG. 7 (hereinafter referred to as “power control”, see Japanese Unexamined Patent Application Publication No. 59-23090). The lower of the flow rates calculated by the two control methods is selected, and a regulator of the pump is instructed to use the flow rate. FIG. 8 illustrates a relationship between the regulator instruction and the pump flow rate.
The flow rate control sets the flow rate at the standby flow rate when the lever is neutral (with consideration of an initial system response) and at the maximum when the lever is fully operated.
In contrast, the power control sets the flow rate at the maximum at the initial load pressure (in an interval A in FIG. 7) and at the minimum at the relief pressure.
Therefore, for a load pressure in the pressure interval A, a pump flow rate set by the flow rate control, which corresponds to a lever operation amount, is selected. For a load pressure higher than the interval A, a pump flow rate set by the power control is selected.
In general, the standby flow rate of the flow rate control is set to be lower than the minimum flow rate of the power control. Therefore, the minimum flow rate of the system is the standby flow rate and does not fall below the standby flow rate.
A hybrid working machine with the above-described structure has the following problem.
As described above, in the hybrid working machine, the maximum engine output is set at a relatively low value (average power), and any shortage in power is covered by using the motor function of the power machine driven by the electric storage device. Thus,maximum pump input>maximum engine output.
Therefore, if a high load state (a state in which the load is equal to or higher than the average power) continues, the level of charge of the electric storage device decreases. When the level of charge falls below a limit, the assistance function of the power machine is lost.
In this case, control for reducing the pump input has to be performed so as to avoid overload. However, with the flow rate control and the power control, the minimum flow rate of the pump cannot be made to be equal to or smaller than the standby flow rate. As a result, the pump input cannot be reduced to a level with which overload can be avoided.
Therefore, a problem exists in that the engine may stall due to overdischarge of the electric storage device (degradation of the electric storage device due to frequent charging/discharging) or due to overload of the engine.